For low and low moderate risk hikes and backpacks, I have no problem taking off solo. Trails, popular areas, and cell coverage all are low risk. I have turned around on both solo day hikes and backpacks where I felt the risk was too great.

Safety with hiking partners are dependent entirely upon the skill and common sense of your partner(s). A partner skilled in land navigation, camp skills, first aid, and having their head firmly planted on their shoulders is a blessing to hike with. I've hiked with others that either been blithering idiots (toted more alcohol then water into the desert) or yahoos that were as likely to end killing themselves as getting you killed trying to rescue them (free climbing up the side of a waterfall).